Project Summary/Abstract Melanoma arises from transformation of melanocytes (MCs) in the basal layer of the epidermis where they are surrounded by keratinocytes (KCs). While research has focused predominantly on identifying driver mutations involved in conversion of MCs to melanoma, histologically normal fields of abnormal KCs are also present in sun-exposed epidermis in which melanomas arise. The extent to which melanoma is promoted by altered KCs that surround early lesions is unknown. Our long-term objectives are to define how altered KC:MC communication drives melanoma development and to identify alterations in KCs surrounding pigmented nevi that serve as predictors of malignant transformation and as therapeutic targets. KC:MC interactions occur directly through cadherins and indirectly through secreted factors. Alterations in classic cadherin expression are known to affect melanoma initiation and progression, but little is known about desmosomal cadherins' roles in melanoma. Our data support the idea that the desmosomal cadherin, desmoglein 1 (Dsg1), is critical for MC homeostasis even though it is expressed only in neighboring KCs. Loss of Dsg1 stimulates the production of KC cytokines and other secreted factors that increase MC number, dendricity, pigment production, and pro- tumorigenic cytokine expression. These features resemble the MC response to ultraviolet (UV) light, and we showed that UV results in selective loss of KC Dsg1. Our data also show that Dsg1 is suppressed by secreted factors from melanoma cells, and its expression is decreased in peri-lesional nests surrounding human melanomas and dysplastic nevi. This suggests the existence of a feedback loop that stabilizes a Dsg1-deficient pro-melanomagenic KC:MC unit within the tumor microenvironment. We propose that Dsg1's loss following UV exposure contributes to MC responses to UV, but that chronic damage and prolonged suppression of Dsg1 promotes melanoma development through coordination of pro-tumorigenic paracrine signaling and altered KC:MC contact. We will use candidate and unbiased analysis of Dsg1-deficient 2/3D human models, human tissues and a novel CRISPR/Cas Dsg1-deficient mouse model in conjunction with an HGF animal model of melanoma that closely resembles human pathology to: 1) Determine how Dsg1 loss alters the KC secretome and stimulates MC behaviors resembling the UV response through cooperative paracrine and cell contact- dependent signaling and 2) Determine how a sustained Dsg1-deficient KC:MC unit is established to promote malignant transformation and melanoma development. Understanding KC:MC communication will provide the basis for development of new biomarkers to identify individuals at high risk of developing malignant melanoma, and who may be candidates for prophylactic treatment with agents that restore normal KC:MC communication.